1. Field of the Invention
This invention relates to a display apparatus such as a liquid crystal display (LCD) apparatus, and more particularly to an active matrix LCD apparatus.
2. Description of the Prior Art
FIG. 13 diagrammatically shows an active matrix LCD apparatus which uses thin-film transistors (TFTs) as switching devices and in which two scanning signal lines are simultaneously scanned (hereinafter, this scanning method is referred to as "two-line simultaneous scanning method"). This TFT active matrix LCD apparatus comprises a large number of scanning signal lines 13 and data signal lines 14 formed on a substrate 11 which cross at right angles, and a matrix array of pixel electrodes 12 connected to these signal lines 13 and 14 via TFTs 15. A common opposite electrode (not shown) is disposed opposite to the TFT active matrix substrate with a liquid crystal layer interposed therebetween. In this construction, when a selection signal is applied to each of the scanning signal lines 13, data signals on the data signal lines 14 are fed via activated TFTs 15 to the pixel electrodes 12 connected to that scanning signal line 13. After the application of the selection signal is completed and the TFTs 15 are inactivated, the data signal potential is retained at each pixel electrode 12 by the capacitance of the liquid crystal layer, etc. The potential is regenerated at each application of the selection signal. Therefore, even in a matrix system in which the selection signal is sequentially applied to the scanning signal lines, data signal potential can be retained at each pixel electrode 12 and applied to the liquid crystal layer.
In the two-line simultaneous scanning method, as shown in FIG. 14, the selection signal is applied to two adjacent scanning signal lines 13 at the same time. When scanning an odd-numbered field, the selection signal is first applied simultaneously to the first and second scanning signal lines 13, and then, after one horizontal scanning period, to the third and fourth scanning lines 13. Thus, the selection signal is sequentially applied to each pair of an odd-numbered scanning signal line 13 and the succeeding even-numbered scanning signal line 13. On the other hand, when scanning an even-numbered field, the selection signal is first applied to the first scanning signal line 13, and then, after one horizontal scanning period, the selection signal is applied simultaneously to the second and third scanning signal lines 13, and thereafter to the fourth and fifth scanning signal lines 13. Thus, the selection signal is simultaneously applied to two adjacent scanning signal lines 13 paired differently from when scanning an odd-numbered field. Accordingly, as compared to a simple scanning method in which the selection signal is applied to one scanning signal line 13 at one time, the two-line simultaneous scanning method requires two times more scanning signal lines 13 and pixel electrodes 12, but can produce a high-resolution image conforming to the interlaced scanning system. The two-line simultaneous scanning method is described in detail in U.S. patent application Ser. No. 07/476,536 filed on Feb. 7, 1990 and EPC patent application No. 90301414.0 filed on Feb. 9, 1990. These are incorporated herein as references.
Upon the completion of the application of the selection signal, the potential at each of the pixel electrodes 12 connected to the scanning signal lines 13 to which the selection signal has been applied drops to a lower level than the data signal potential because of the effect of a parasitic capacitance C.sub.gd between the gate and drain of the activated TFT 15. In the case of the simple scanning method, when the gate voltages at the activation and inactivation of the TFT 15 are denoted as V.sub.GH and V.sub.GL, respectively, and the capacitance of the liquid crystal layer at the pixel electrode 12 is denoted as C.sub.LC, the potential drops below the data signal potential approximately by the potential .DELTA.V indicated by the following expression (1). ##EQU1##
In the simple scanning method, however, such a potential drop occurs equally to every pixel electrode 12. Therefore, by shifting the opposite voltage applied to the opposite electrode by a value equivalent to the potential drop .DELTA.V, the DC component of the voltage applied to the liquid crystal layer by the AC drive can be easily maintained at zero.
In the two-line simultaneous scanning method, on the other hand, the effect of a stray capacitance C.sub.pg between the pixel electrode and a scanning signal line not connected but adjacent to the pixel electrode must also be considered in addition to the parasitic capacitance C.sub.gd. That is, when selection signals Sa and Sb are applied to scanning signal lines 13a and 13b, as shown in FIG. 15, thereby selecting the rows to which pixel electrodes 12a and 12b are connected, the pixel electrode 12a disposed between the scanning signal lines 13a and 13b and connected to the scanning signal line 13a experiences a potential drop .DELTA.V1 indicated by the following expression (2), when the application of the selection signal Sa is completed. This is because the scanning signal line 13b which is not connected but adjacent to the pixel electrode 12a also experiences a potential variation as the selection signal Sb is applied to it. ##EQU2##
However, regarding the other pixel electrode 12b, since the selection signal is not yet applied to the scanning signal line 13c not connected but adjacent to the pixel electrode 12b and therefore no potential variation occurs, the potential drop .DELTA.V2 at the pixel electrode 12b is indicated by the following expression (3). ##EQU3##
As a result, although the pixel electrodes 12a and 12b are connected to the same data signal line 14 and supplied with the same data signal voltage, a difference arises between the potential drops .DELTA.V1 and .DELTA.V2 when the application of the selection signal is completed, and thereafter the pixel electrode 12b retains a higher potential.
Accordingly, a prior art liquid crystal display has the problem that when the selection signal is applied simultaneously to a plurality of scanning signal lines, there occurs a difference in brightness between adjacent pixels on the same data signal line, resulting in a degradation in the image quality.